Along with recent improvements of image qualities of color printers and color copying machines, it becomes very easy to copy and faithfully reproduce an original image as a print. This makes it easy to forge securities such as bank notes, and various measures against such forgery have been taken.
Conventional anti-forgery techniques are roughly classified into a technique of recognizing an original as securities and inhibiting faithful printing and a technique of attaching, to a print, machine identification information for identifying a machine so as to specify a machine used for forgery of securities.
Especially, for the latter technique of multiplexing machine identification information onto image information, various methods have been proposed and applied to not only anti-forgery of securities but also copyright protection, secret information protection, and character/voice data transmission.
Methods of multiplexing information of a different kind such as machine identification information onto image information are roughly classified into a method of embedding information in a frequency domain and a method of embedding information in a real space domain. Embedding in a frequency domain is mainly implemented by converting image data from a real space domain into a frequency domain using a means such as FFT (Fast Fourier Transform) and then multiplexing additional information using a frequency component or phase component. On the other hand, embedding in a real space domain is implemented using quantization error which occurs when image data or the value of the least significant bit (LSB) of each pixel in the image data is quantized.
For the shape of a code to be attached, for example, a method of attaching a dot pattern formed from a plurality of regions with different longitudinal directions has been proposed.
However, the code used in the above method has a shape with a specific longitudinal direction. For this reason, when, e.g., image information is processed by pseudo-halftoning processing using error diffusion, and this code is attached to the image information, a continues dot layout appears in a specific direction in a highly dispersible dot layout unique to the error diffusion. Hence, the code is visually noticeable, resulting in degradation in image quality.
To attach identification information for anti-forgery, information blocks each having a plurality of codes must be laid out in the entire image. As a consequence, the codes tend to be periodically laid out and are visually perceivable, resulting in degradation in image quality.
For the shape of a code representing information, another method has been conventionally proposed in which in attaching, as a code, a dot pattern to an image expressed by pseudo-halftoning using error diffusion, a dot pattern that can appear in a flat image having an image density different from that of the region where the code is to be attached as a result of pseudo-halftoning expression using error diffusion is employed as a code shape, thereby suppressing degradation in image quality.
In this conventional method, however, the code shape is represented by a dot pattern having a plurality of pixels in the vertical and horizontal directions. The dot pattern is registered in advance, and appropriately selected and used in accordance with the image density in the prior art using error diffusion.
To attach a code with less degradation in image quality, a dot pattern representing the code is selected in accordance with the density of a region where the code is to be attached. However, having a number of patterns increases, e.g., the number of program codes, hardware configuration, or memory capacity.
In the above prior art, a dot pattern is generated in accordance with the average density of a region having a plurality of pixels, and used as a code. To do this, at the time of processing for the start row where code generation starts, the pixel values of rows to be processed later must be referred to. However, to prevent an increase in memory capacity and increase the processing speed, pseudo-halftoning processing may limit pixels to be referred to at once to one row. In this case, the image density can be detected only in the processing row, and the average pixel value of the entire region where the code is to be attached cannot be obtained.
As another technique of multiplexing information of a different kind in an image processing apparatus for expressing an image using pseudo-halftoning processing, for example, in pseudo-halftoning processing using error diffusion or the like, a combination of quantization values that cannot occur is artificially generated, maintaining the image density, whereby information of a different kind is multiplexed while suppressing degradation in image quality.
However, the above prior art has the following problems.
(1) If information is multiplexed on a frequency component of image data or pixel value of full-color image data, a printing apparatus for printing an image using pseudo-halftoning expression do not guarantee that a code representing the multiplexed information should be preserved on a printing medium such as printing paper.
(2) A recent printing apparatus has high resolution to improve the image quality. To detect a code embedded in an image, an image reading apparatus such as a scanner must also have high read resolution. To reliably read a code, the reading apparatus is required to have resolution higher by twice or more than that on the printing side due to the sampling theorem. However, the read resolution of an image reading apparatus such as a popular scanner is equal to or lower than the resolution of a printing apparatus such as an inkjet printer. Hence, if information of a different kind is multiplexed with image information without considering the resolution of the reading apparatus, an expensive special scanner with high resolution must be used to reliably separate the information of the different kind from the printed image, resulting in impracticality.
To control attachment of a code representing information, a method has been conventionally proposed in which in printing using achromatic colors that are not used for forgery of securities, no anti-forgery information such as the machine number of a printing apparatus is not attached.
However, in the above-described code attachment control method, a code is attached to an output image containing chromatic colors even when the information cannot be decoded due to the image state. For this reason, a code is attached as long as the image contains chromatic colors, resulting in a decrease in printing speed in many cases.
This influence is especially large when code attachment processing is performed not by hardware having relatively high processing speed but by software such as a printer driver.
The outline of additional information superposition processing will be described. FIG. 104 is a block diagram showing a general additional information embedding method. Referring to FIG. 104, an image signal represented by RGB components and input from a terminal 101 is converted into four components: C (cyan), M (magenta), Y (yellow), and K (black) by a color conversion section 102. These components are corrected by a various correction processing section 103. Next, a pseudo-halftoning processing section 104 executes pseudo-halftoning processing using a method such as systematic dithering or error diffusion.
Additional information generated by an additional information generation section 105 is superposed (added) on the Y component of the image signal that has undergone the above processing operations. When these components are input to a printer engine 106, an image having certain information other than image information can be printed.
FIG. 105 is a view showing a yellow plane to which a specific dot pattern is attached in order to superpose additional information. The hatched region (×[inch]) shown in FIG. 105 is called an information area. The additional information is represented by the layout of dots in this region.
However, the above-described method has the following problem.
The above-described dot pattern attachment processing degrades the image quality because information that is not present on the image is attached.
However, the degree of degradation in image quality largely depends on the texture of the image. More specifically, when the image to which additional information is to be attached is a very complex image containing a number of high-frequency components, degradation in image quality due to dot pattern attachment can hardly be visually confirmed.
To the contrary, when the image is a relatively simple image containing a number of flat portions, an unnatural yellow dot may be easily visually confirmed.
Conventionally, the degree of complexity of an image is detected using a certain method. Preferably, if it is determined that the image is complex, a dot pattern is attached as in the prior art, and if it is determined that the image is simple, the amount of dot pattern attachment is decreased as compared to the prior art. However, such a method has not been proposed yet.
Additionally, when a predetermined dot pattern obtained by simple addition is attached, as in the above-described additional information superposition processing method, the number of pixels changes in the neighboring region. More specifically, a change in number of pixels on an image that has undergone pseudo-halftoning processing means a change in density. If this occurs, an unnatural dot may be visually confirmed even if a yellow component can hardly be visually detected.
To avoid this situation, in attaching a dot pattern onto an image signal that has undergone pseudo-halftoning processing, certain processing must be executed to store the density (the number of pixels) in accordance with the situation of the neighboring portion.
A method has been proposed in consideration of this problem, in which processing is executed for image information before pseudo-halftoning processing. However, a method of attaching a dot pattern to image information after pseudo-halftoning processing while storing the density has not been proposed.
In a general method of superposing additional information onto an image, matrix points present at an N-pixel interval in a printable region are defined, as shown in FIG. 106, and additional information is expressed depending on whether a dot pattern predetermined and stored in a storage device in a computer or printer main body for executing processing is present at each matrix point.
In the conventionally proposed scheme, generally, the same dot pattern is used for all density regions on an image. Attachment of a dot pattern onto image information using this conventional scheme will be described with reference to the accompanying drawings. Assume an image after pseudo-halftoning processing as shown in FIG. 107 as an image example before superposition of additional information. When a dot pattern shown in FIG. 108 is attached to the image shown in FIG. 107, an image shown in FIG. 109 is obtained.
Referring to FIG. 109, in a density region (especially region A) having a pixel layout similar to that of the dot pattern, it is difficult to determine whether the dot pattern is present in the process of reading the attached information, and therefore, accurate additional information cannot be read.
To avoid this situation, the dot pattern may have pixels at a smaller interval, as shown in FIG. 110. However, when the dot pattern having pixels at high density as shown in FIG. 110 is attached to the entire image, an unnatural dot may be visually confirmed in a region having very low density, conversely to the above case, although a yellow component is hard to visually detect.
To avoid this state, in attaching a certain dot pattern to image information that has undergone pseudo-halftoning processing, processing of measuring the density, i.e., pixel density in the region around the attachment portion by some method and attaching an appropriate dot pattern in accordance with the measurement result must be executed. However, such a method has not been proposed yet.
FIG. 111 is a view showing an example of a yellow plane to which a specific dot pattern is added to superpose conventional additional information. Referring to FIG. 111, a dot indicates a point where a dot pattern shown in FIG. 112 is attached. FIG. 113 is an enlarged view of FIG. 111. Referring to FIG. 113, a pixel represented by a dot is a pixel (to be referred to as an “on-dot” hereinafter) printed on a printing medium such as paper using ink or toner.
In the above-described method, the dot pattern is generally embedded in the entire image at a predetermined period, as shown in FIG. 111. This predetermined period is an A-pixel period in the main scanning direction, and a B-pixel period in the sub-scanning direction.
However, the above-described conventional method has the following problem.
FIGS. 114A to 114D are views showing CMYK components after pseudo-halftoning processing. Especially, the yellow plane shown in FIG. 114D indicates Y (yellow) components of the CMYK components. The dot pattern shown in FIG. 112 is attached to each of halftone dot regions. It is found that, in all the CMYK planes, neither region A nor B have on-dots.
The conventional additional information superposition means attaches a dot pattern to all attachment positions on the Y plane, as shown in FIG. 115, regardless of the situation.
For this reason, in a region such as the region A or B where no on-dots are present in all the CMYK planes, only pixels of the dot pattern are printed on a printing medium such as paper. Under these circumstances, an unnatural dot may be visually confirmed, though the yellow component is hard to visually detect.
To avoid this situation, for example, in attaching a dot pattern onto the Y plane, processing of measuring, by some method, the density state (pixel distribution) around the position where the dot pattern is to be attached for all the CMYK components and determining in accordance with the measurement result whether the dot pattern should be attached is executed.
Even when this processing is executed, the attached information can be restored because the dot pattern representing the presence of additional information is periodically present. However, such a method has not been proposed yet.